An ultrasound diagnosis device is a device that transmits an ultrasound pulse from an ultrasound probe into a biological body and receives an ultrasound echo scattered or reflected from the biological body using the ultrasound probe to perform various signal processings on the received ultrasound echo to obtain a body tissue B-mode image or a blood flow image and is widely used for medical diagnosis.
One of imaging methods of the ultrasound diagnosis device is an ultrasound contrast imaging using an ultrasound contrast agent. The ultrasound contrast imaging is a method that intravenously injects an drug formulation, which is obtained by stabilizing microbubbles having a micron size order using some method, as an ultrasound contrast agent into the biological body in advance to perform the ultrasound imaging. This method has been widely used to diagnose a disease of blood vascular system such as malignant tumor or infarction.
As for ultrasound wave of several MHz which is mainly used for ultrasound diagnosis, the ultrasound contrast agent in forms of microbubbles shows significantly high nonlinear response. Therefore, a nonlinear component of the ultrasound echo in the ultrasound contrast imaging includes lots of ultrasound echoes from the ultrasound contrast agent. There is an attempt that the ultrasound echo of the nonlinear component is extracted to form an image to visualize a vascular structure. The related art will be described in detail with reference to FIGS. 2 and 3.
FIG. 2 is a frequency spectrum illustrating both a frequency band of a transmit pulse and a received echo of an ultrasound wave and a sensitivity frequency band of an ultrasound probe in the related art.
If the transmit pulse 200 is transmitted from the ultrasound probe of the probe sensitivity band 10 to perform the imaging by the ultrasound contrast imaging, a linear tissue echo component 300, nonlinear tissue echo components 400 and 500, and an contrast echo component 600 are received so as to be included in the received echo. Here, the linear tissue echo component 300 is an echo received from a tissue by a fundamental component of the transmit pulse 200. Further, the nonlinear tissue echo component 400 is a received echo from a tissue by a second harmonic wave component (sum frequency component of a frequency component of the fundamental wave included in the transmit pulse 200) which is produced during the process of propagating the transmit pulse 200 in the biological body. Like the nonlinear tissue echo components 400, the nonlinear tissue echo component 500 is a received echo from a tissue by a nonlinear component which is produced during the process of propagating the transmit pulse 200 in the biological body and generated by a difference-frequency component of the frequency components of the fundamental wave included in the transmit pulse 200.
The contrast echo component 600 is widely distributed with a high acoustic intensity in a frequency range of the probe sensitivity band 10 by a strong nonlinear response of the contrast agent. However, the nonlinear tissue echo components 400 and 500 are generated by a nonlinear acoustic effect (waveform distortion or accumulation thereof) of the biological tissue. Therefore, a ratio of the intensities of the nonlinear tissue echo components 400 and 500 to the linear tissue echo component 300 is low. Therefore, by processing a frequency band where the nonlinear tissue echo components 400 are distributed using a filter represented by a pass band 40, a frequency component in which only an intensity of the contrast echo component 600 is comparatively strong may be extracted so that the imaging is performed with a signal of the extracted frequency component. By doing this, it is possible to visualize a vascular image which is not comparatively buried in the signal from the tissue. In this related art, it is required to sufficiently separate the frequency band in which the linear tissue echo components 300 are distributed and the frequency band in which the nonlinear tissue echo components 400 are distributed and a lower frequency band of the probe sensitivity band 10 is not necessarily used for imaging.
Next, another related art will be described with reference to FIG. 3.
FIG. 3 illustrates a method that adds a received echo R1 obtained by a first transmit pulse 210 and a received echo R2 obtained by a second transmit pulse 211 produced by positive-to-negative inversing the first transmit pulse 210 and transmitted on the same scanning line as the first transmit pulse 210 to remove an echo reflected from the tissue by a fundamental component of the transmit pulse and is referred to as pulse inversion technique.
The received echo R1 from the first transmit pulse 210 includes a linear tissue echo component 310 from a tissue by a fundamental component of the transmit pulse 210, nonlinear tissue echo components 410 and 510 from a tissue by a nonlinear component of sum frequency component and difference-frequency component generated in the process of propagation of the first transmit pulse 210 in a biological body, and a contrast echo component 610. Further, the received echo R2 from the second transmit pulse 211 includes a linear tissue echo component 311 from a tissue by a fundamental component of the transmit pulse 211, nonlinear tissue echo components 411 and 511 from a tissue by a nonlinear component of sum frequency component and difference-frequency component generated in the process of propagation of the second transmit pulse 211 in a biological body, and a contrast echo component 611.
By adding the received echo R1 and the received echo R2, the linear tissue echo components 310 and 311 are removed because of the linear process. Finally, the contrast echo component 612 obtained by adding the contrast echo components 610 and 611, a sum frequency tissue harmonic echo component 412 obtained by adding the sum frequency tissue harmonic echo components 410 and 411 and a difference-frequency tissue harmonic echo component 512 obtained by adding the difference-frequency tissue harmonic echo components 510 and 511 are extracted.
According to this pulse inversion technique, intensities of the sum frequency tissue harmonic echo component 412 and the difference-frequency tissue harmonic echo component 512 are smaller than the intensities of the removed linear tissue echo components 310 and 311 for the fundamental wave and a signal component of the contrast echo component 612 widely included in the probe sensitivity band 10 is used to form an image. Therefore, it is possible to construct a vascular image with the contrast echo which is not comparatively buried in a wide band signal from the tissue.
As described above, in the ultrasound contrast imaging, it is important to increase an intensity ratio of the echo from the ultrasound contrast agent and the echo from the body tissue (referred to as a contrast-to-tissue ratio or CTR). Therefore, in addition to the above-mentioned related art, a method of increasing a CTR is disclosed as follows.
For example, in Patent Literature 1, a method that performs a transmit and receive sequence in which an amplitude and a phase are controlled for the same scanning line three times or more at the time of transmission and reception, respectively to suppress a tissue echo component is disclosed. According to the related art, as an example of three transmit and receive sequences, first transmission and reception is performed by a transmit pulse P1 having an amplitude of 1 and a phase of 0 degree, second transmission and reception is performed by a transmit pulse P2 having an amplitude of 2 and a phase of 180 degrees, and third transmission and reception is performed by a transmit pulse P3 having an amplitude of 1 and a phase of 0 degree and the three received echoes are added with a weight of 1:1:1. As a result, a linear component of a tissue echo is suppressed, but an echo from a contrast agent shows a nonlinear response with respect to the amplitude or the phase of the transmit pulse, and thus is not suppressed by the addition. Therefore, the contrast echo and the tissue echo are intended to be separated. The main difference from the related art described with reference to FIGS. 2 and 3 is that the CTR is increased by aggressively using the contrast echo component with respect to the fundamental component of the transmit pulse.
As another example of a method of increasing a CTR, in Patent Literature 2, a method that increases the CTR by transmitting and receiving two times or more a transmit pulse in which a frequency band of a sum frequency tissue harmonic echo component is generated out of a probe sensitivity band on the same scanning line and aggressively using the contrast echo component for the fundamental component of the transmit pulse is disclosed. The two or more transmit pulses are differently modulated in at least one of an amplitude, a phase, and a polarity and the linear tissue echo component and the sum frequency tissue harmonic echo component are suppressed to increase the CTR.